In general, the fundamental mechanism of photodynamic therapy (PDT) action is initiated by the absorption of visible light by a tumor that has been injected with a photosensitizing agent. As a result of the reaction of incident light in the range of 600 to 1000 nanometers (nm) with the photosensitizing agent, oxygen is generated in the singlet state. The reaction of this photochemically generated singlet oxygen with the intracellular lipids, proteins, and nucleotides harms the cancerous cells and ultimately results in tumor necrosis.
In the prior art, lasers and continuous wave (CW) noncoherent light sources were used for PDT. The delivery dose of the light energy varied in the range of 25 to 200 joules per square centimeter (J/cm.sup.2) with a fluency rate of 10 to 200 milliwatts per square centimeter (mW/cm.sup.2), depending upon the tumor size and the spectrum of the radiation.
Typically, the light source used for clinical PDT of solid tumors is a CW tunable dye laser that is pumped by an argon ion laser. Alternatively, a frequency doubled Nd:YAG pumped dye laser which produces pulsed light may be used.
As an alternative to laser light sources, CW wavelength filtered lamp sources may be used. For example, the use of a xenon short arc lamp of 150 W possessing a narrow light beam with a spectral region in the range of 610 to 750 nm is claimed in United States Patent No. 5,344,434, issued on Sept. 6, 1994, to Eli T. Talmore, entitled "Apparatus For The Photodynamic Therapy Treatment." A glass lens focuses the light beam to a range of 3 to 12 millimeters (mm) in diameter. The light beam is then delivered to the target through a light guide. To obtain the desired dose of radiation with this device, the treatment time preferably ranges from approximately 20 to 60 minutes, depending on the size of the tumor.
In the prior art, one cancer diagnostic method that is used is based on the fluorescence of malignant cells in the wavelength range of 400 to 750 nm under illumination of near ultraviolet (UV) and blue radiation in the range of 300 to 400 nm. The presence of malignant cells reduces the autofluorescent intensity in the blue green wavelength region, thus providing a signal that is distinguishable from the higher intensity signal originating from surrounding healthy tissue. Alternatively, diagnostic techniques may employ different types of injected photoactivators. Accumulation in the tumor of these alternate types of photoactivators results in increased fluorescence of the malignant cells in comparison to the surrounding healthy tissue.
Only a few types of light sources that can excite tissue autofluorescence are currently available. One example of such a light source is a nitrogen laser which emits 3 nanosecond (nsec) light pulses having a 337 nm wavelength. Another type of light source is a UV source, such as an excimer pumped dye laser, that produces a 308 nm light beam which is focused into a 600 micrometer (.mu.m) thick optical fiber. As an alternative to the use of lasers as the exciting light source, mercury lamp sources which filter two excitation wavelengths of 365 and 405 nm may be employed.
There is, however, a need for a simple tunable apparatus and method for providing efficient PDT treatment for a wide range of tumor parameters, including size of the tumor and depth of location. Such an apparatus and method will preferably be able to control the fluency rate and spectrum of output radiation, dependent on the type of photosensitizing agent used, to achieve efficient PDT treatment. In addition, the apparatus and method preferably will produce radiation with the appropriate wavelength range desirable for cancer diagnostics. In addition, an apparatus and method using pulsed light rather than CW light would be advantageous because pulsed light enables better temperature control and the achievement of important hyperthermion effects.